Evaporation induced self-assembly and characterization of gold nanorods

Abstract

Assembly of nanoparticles into superstructures in suspension and at interfaces exhibits many different and interesting features. In this thesis we present results of different studies into the assembly of gold nanorods. We describe the synthesis of low aspect ratio gold nanorods as used in this thesis, and review their optical properties. The role of various chemicals used in the synthesis of nanoparticles is also addressed. Nanoparticles and their assembled superstructures can be characterized by a range of different techniques. Imaging techniques such as scanning electron microscopy (SEM) are widely used, while more recently also other charged particle imaging have been developed, including helium ion microscopy (HIM). We compare the aforementioned imaging tools as applied to our surfactant-coated gold nanorods. The surfactant, cetyltrimethylammonium bromide (CTAB) has a profound effect on the images in HIM, while in SEM this layer is much less visible. From a careful analysis of the HIM and SEM results, we determine the thickness of the surfactant layer and relate it to the CTAB molecule length. We study the deposition of CTAB stabilized gold nanorods on HOPG surfaces using SEM, STM and AFM techniques. The well-defined nature of the substrate, in terms of step edges and terraces gives rise to assembly of the nanorods specifically at the step edges. Moreover, the excess CTAB in solutions leads to self-assembled striped layers on the HOPG surface, which we also characterize in terms of periodicity and dynamic behaviour. We investigate the phase separation of gold nanospheres and nanorods on homogeneous unpatterned silicon surfaces during evaporative assembly. The phase separation occurring in the well-known coffeestain ring is discussed in terms of interparticle interactions related to the particle shape and size. In the last two chapters, we focus on the evaporative assembly on chemically stripe-patterned surfaces consisting of alternating hydrophilic/hydrophobic stripes having widths in the micrometer range. More specifically, we focus on stripe-patterned surfaces with a wettability gradient, which allows to control the motion of evaporating droplets and their contact lines. The deposition and assembly of nanoparticles (rods and spheres) preferentially on the hydrophilic regions are presented and discussed, also in relation to the results obtained on non-patterned surfaces. Finally, the hydrodynamic confinement of gold nanorods and their alignment induced by the motion of a contact line on radial gradient patterned surfaces. The 2D order parameter within narrow deposits on the hydrophilic stripes is analyzed in relation to the dimensions of the single layer deposits.